This application is based on applications Nos. 9-7979 and 9-92582 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an apparatus for and a method of measuring a distribution of luminous intensity along each direction of a spot light source such as a halogen lamp or a surface light source such as a CRT or a liquid crystal display (hereinafter, these light sources are collectively referred to as "sample light sources (light sources to be measured)"). The present invention also relates to an apparatus for and a method of measuring a distribution or spectral distribution of luminous intensity along each direction of a spot light source such as a halogen lamp, a partial area of a surface light source such as a CRT or a liquid crystal display, or a fine area illuminated by an illumination means (hereinafter, these light source and areas are collectively referred to as "measurement area") for each wavelength.
2. Description of the Background Art
A distribution of luminous intensity is one of the important light source characteristics, and a variety of apparatuses for measuring the distribution of luminous intensity have been proposed. For example, Japanese Examined Patent Publication No. 3-4858 discloses a luminous intensity distribution measuring apparatus in which a sample light source is arranged at a front focal point of an f.multidot..theta. lens, an aerial image of the sample light source is formed on a plane where a rear focal point of the f.multidot..theta. lens is located, and an image of the distribution of luminous intensity (hereinafter, "luminous intensity distribution image") of the sample light source is observed by directing the aerial image on a sensing surface of a television image pickup device via a relay lens system.
A measurable range is crucial in measuring the distribution of luminous intensity of the sample light source. The distribution of luminous intensity characteristic of the liquid crystal display as one of the sample light sources has been improved in recent years. In order to estimate this improved liquid crystal display, it is necessary to measure a distribution of luminous intensity over a range of at least .+-.80.degree. with respect to a normal line to each fine area (measurement area) of the liquid crystal display.
However, since the luminous intensity distribution image of the sample light source is formed using the f.multidot..theta. lens in the above prior art, the measurable range is inevitably limited to a range of .+-.60.degree. with respect to the normal line to the measurement area of the sample light source. It is difficult at present to meet a recent demand for the luminous intensity distribution measuring apparatuses.
Further, since the transmittance of polarized light components of an incident light on a lens varies depending on an incidence angle as has been known, it is difficult to conduct an accurate measurement by the prior art which measures a distribution of luminous intensity via the f.multidot..theta. lens. For example, if light having the same luminous intensity is emitted from the measurement area at different angles .theta.1, .theta.2 with respect to the normal line to the measurement area, the intensity of the lights transmitting the f.multidot..theta. lens changes according to the angles .theta.1, .theta.2 due to the nonuniform transmittance. This problem is particularly evident if the sample light source is a liquid crystal display which is a light source for irradiating polarized lights.
There is also known a method for measuring a distribution of luminous intensity of the sample light source by arranging a photodetector such as a multiplier phototube or photodiode in a position distanced from the sample light source and by detecting the luminous intensity in the respective moving positions while the photodetector is successively moved along a concentric circle with respect to the sample light source. According to this method, the above problem can be solved, but another crucial problem is caused.
Specifically, in order to conduct a luminous intensity distribution measurement with high accuracy according to this method, it is necessary to increase measurement points by repeatedly moving the photodetector and the measurement many times. Accordingly, it takes a longer time to conduct a measurement. Further, since the measurements cannot be simultaneously conducted at all measurement points according to this method, it is requisite that the sample light source be stable from the start to the end of the measurement. Actually, it is impossible to measure the distribution of luminous intensity of the sample light source with high accuracy.
Furthermore, in order to estimate a CRT or liquid crystal display for displaying a color image, it is important to measure a distribution of luminous intensity which includes a color information and is actually observed by a human being, in other words, a spectral distribution of luminous intensity.